Bell Ringer 1-26-10 In your own words, write a complete sentence defining the word “force.”

Force Chapter 10 Sections 1 and 2

Force A force is a push or a pull. Like velocity and acceleration, a force is described by its strength AND direction. The SI unit for force is the newton ( N ).

Net force Net force = the combination of ALL the forces acting on an object. The net force determines whether an object moves and in what direction.

Combining Forces   = -- 5 N 5 N 10 N   = -- 5 N 5 N 10 N Add forces acting in the same direction  -- =  5 N 10 N 5 N Subtract forces acting in opposite directions. The force goes in the direction of the greater force.   = 0 5 N 5 N If forces are equal and they go in opposite directions, they can cancel each other out.

Balanced Forces Equal forces acting on one object in opposite directions = balanced forces. Each force is balanced by the other. When forces are balanced there is NO movement. (NO net force)

Unbalanced forces Unbalanced forces are NOT balanced and this causes an object to start moving, stop moving, or change direction. Unbalanced forces result in a net force and cause a change in the object’s motion.

Friction Friction= the force that two surfaces exert on each other when they rub against each other. The strength of friction depends on (1) how hard the surfaces push together and (2) the types of surfaces involved.

Friction Foldable Once your paper is folded, label the four sections with the four types of friction: Static friction Sliding friction Fluid friction Rolling friction p. 342

Types of friction Static friction – friction that acts on objects that are not moving Ex. Pushing on a heavy refrigerator/ driving off with a book on top of the car Sliding friction – friction that occurs when 2 solid surfaces slide over each other Ex. Bike brakes/ ballet dancers

Types of Friction (cont’d) Rolling friction- occurs when an object rolls across a surface (ex: skateboard on road) Fluid friction – occurs when a solid object moves through a fluid (ex: surfboard on water)

Exit Quiz 1-26-10 What is a force? The combination of all the forces acting on an object is called ______. If two forces are balanced, is there a net force? The SI unit for force is the ________. Tell me one of the four types of friction and give an example.